In some radiofrequency information transmission applications it has been observed that the transmit or receive antenna might have an impedance depending strongly on conditions external to the antenna, and depending notably on the environment in which the antenna is placed.
For example, in medical telemetry it is possible for an antenna in a probe to be placed inside the human body, and the impedance then depends strongly on the biological environment in which the antenna finds itself. It depends on the electrical properties (conductivity, dielectric constant) of the surrounding tissues (muscles, fat) or on the liquid environment (blood, other fluids) in which the antenna may be immersed.
Even in more conventional radiofrequency transmission applications (mobile telephony, etc.) the impedance of the antenna can vary.
A transmission system (or reception system respectively) comprises at least one amplifier with which one or more filters may be associated.
Generally speaking, the variations in antenna impedance are particularly sensitive for very small-sized antennas having a high quality factor, used in applications with strong miniaturization constraints.
These variations in impedance may lead to losses called mismatch losses: these losses result from the fact that the transmission system that supplies the antenna, or the reception system that receives a signal from the antenna, is generally designed for optimal performance when it is loaded (at output for the transmission system or at input for the reception system) with a well determined nominal impedance. Its performance deteriorates when it is loaded with an impedance different from its nominal value. The mismatch losses can be up to 40 dB.
This is why attempts have already been made to put an impedance matching network between the output of the transmission system and the transmit antenna (and this can also be done at the input for a receive antenna), which has the effect that the transmission system sees a different impedance from that of the antenna and which is preferably equal to the nominal value for which it was designed, for example 100 ohms or 500 ohms. The matching network is tuneable, i.e. its capacitive and/or inductive elements have adjustable values in order to take account of the conditions in the environment of the antenna so that the matching is the best possible, whatever the circumstances.
In the prior art illustrated by patent U.S. Pat. No. 4,375,051 it has been proposed to use a bidirectional coupler to detect a mismatch: the amplifier power is applied to a load by means of the coupler and of an impedance matching network. If the assembly is mismatched, a part of the power sent to the antenna is reflected by it instead of being sent into the surrounding environment. The reflected part passes back into the coupler and exits it through a specific output. The reflected output power is detected, measured and serves to servo-control the impedance network in order to change its constitution in a direction tending to reduce the reflected power. The matching network comprises variable capacitors.
This method suffers from two drawbacks. On the one hand, the reflected power may be low and subject to parasitic interference as all interference picked up by the antenna distorts the measurement due to the fact that it is added to the reflected power while it does not constitute power reflected by the mismatch. On the other hand, there is no one-to-one relationship between the quantity of reflected power (which serves as input to the servo-control) and the complex impedance value which it has to provide to the matching network in order to truly match the amplifier to the antenna. This method therefore leads to an new impedance which is not necessarily optimal, as several complex impedance pairs correspond to a given power. This is because detecting the mismatch through the power measurement (scalar method) causes a loss of information about the phase shift between the currents and voltages. The known method therefore employs a slow servo-control system, functioning by successive iterations and trial and error, without simply converging on a stable match position. Finally, the bidirectional couplers it uses are bulky. These systems are not well suited for applications where the matching speed and size are important criteria. According to the invention, simultaneous detection of the amplitude and the phase of the current and of the voltage at the output of the transmission system (or at the input of the reception system) is proposed. The mismatch detection is therefore vectorial and the ratio of the voltage to the current is truly representative of the impedance Z seen by the system loaded by the assembly of the matching network and of the antenna of impedance Zant, on condition that the voltages and currents are considered in vector form, i.e. by taking account of the phase shift between current and voltage. The load impedance seen by the amplifier is a function of the antenna impedance Zant and the various impedances of the matching network; the antenna impedance Zant is then calculated from the measured load impedance Zm and impedances of the matching network, the configuration of which is known at the moment of measurement, and finally the modification that needs to be applied to one or more of the impedances of the matching network is calculated in order that the impedance seen by the amplifier becomes matched to the nominal impedance of the amplifier in the current conditions of the environment of the antenna.
The invention therefore enables direct calculation of the optimum impedance matching network in a mathematical, hence certain and quick, manner.